from my past experience working as a ramp agent, when pax need to be offloaded there are a few options we go through, depending why the offload needs to be done:
1. always ask for volunteers first
2. last 10 pax to check in = offloaded.
3. cheapest paid fare = offloaded
a few more that cant think of right now
if its a weight issue, pax are the very last to be offloaded normally. every airline is different but its normally:
1. standby/rebate staff travel pax
2. catering
3. baggage
4. then passengers.........
you are right to ask for an explantion, i would keep asking until you get one. it is not right them offloading two memebers of your family. i hope you get a reply soon from them.